The present invention relates to a pulse width modulation amplifier used for amplifying the output power of acoustic reproduction audio devices, wherein the output offset voltage and output noise are reduced.
The conversion efficiency of pulse width modulation amplifiers is higher than that of typical class A, AB, and B amplifiers. For example, the conversion efficiency of a class B amplifier, which is generally characterized as having good conversion efficiency, is around 70%. On the other hand, the conversion efficiency of a pulse width modulation amplifier can be 85-90%. Accordingly, pulse width modulation amplifiers are widely used as low frequency amplifiers in audio devices in vehicles or in public address systems.
An example of a conventional pulse width modulation amplifier is illustrated by the block diagram shown in FIG. 4. The pulse width modulation amplifier comprises an operational amplifier, used as a direct current amplifier, a pair of feedback circuits 2 and 3, a triangular wave oscillator 4, a comparator 5, an output stage driving circuit 6, a final output stage 7, low-pass filters 8, and a load 9.
A pulse width modulation signal is generated by the comparator 5 by comparing an input signal amplified by the operation amplifier 1 with a triangular wave signal generated by the triangular wave oscillator 4. The pulse width modulation signal is amplified by the output stage driving circuit 6 to drive the final output stage 7. The output of the final output stage 7 is demodulated, removing the carrier wave and higher harmonic by the low-pass filters 8, resulting in a low frequency output signal supplied to a load 9.
Field-effect transistors ("FETs") are used in the output stage driving circuit 6 and the final output stage 7 since, compared with transistors, FETs have a low driving current and exhibit a low resistance when turned on. The FETs in the final output stage 7 are generally connected in a H-bridge configuration.
A pair of negative feedback loops are provided between the final output stage 7 and the operational amplifier 1, for improving the distortion factor, noise, etc. in an audio circuit.
In these conventional pulse width modulation amplifiers, the carrier of the same frequency (hereinafter carrier) as the oscillation frequency of the triangular wave oscillator 4 must be eliminated before the signal from the final output stage 7 is negatively fed back to the operational amplifier. Low-pass filters are included in the feedback circuits 2 and 3 to eliminate the carrier.
The details of the circuit elements around the operational amplifier are shown in FIG. 5. The operational amplifier 1 comprises a direct current-differential amplifier, capacitors 11,13 and 15 and resistors 12,14,16 and 17.
An input signal is applied to the input terminal 10, passed through a capacitor 11 and a resistor 12, and applied to the non-inverting (positive) input of the operational amplifier 1. Signals from the output stage 7 are fed back to the non-inverting (positive) and the inverting (negative) inputs through the feedback circuits 2 and 3. The capacitors 13 and 15 limit the high frequency so that high frequency is not applied to the operational amplifier from the feedback circuits 2 and 3. Frequencies higher than a cut-off frequency Fc are not negatively fed back. The cut-off frequency Fc is generally around 20 kHz. The cut-off frequency is determined according to the time constant (1/2.pi. CR) when C is the capacitance of capacitor 13 or 15 and R is the resistance (not shown in FIG. 5) of the feedback circuits 2 and 3.
The feedback circuits 2 and 3 are shown in detail in FIG. 6. The circuit shown in FIG. 6 is essentially the same as the circuit shown in FIG. 5 in that the circuit includes operation amplifier 1 as a direct current-differential amplifier, capacitors 13 and 15 and resistors 12,14,16 and 17. The carrier and the higher harmonics in the signals fed back from the output stage 7 are removed by a pair of low-pass filters. One low-pass filter is formed by a resistor 19 and a capacitor 20. The other low-pass filter is formed by a capacitor 22 and a resistor 23. After passing through the low-pass filters the signals are fed through the resistors 18 and 21 to the positive and negative inputs, respectively, of the operation amplifier 1.
The ground points of the capacitors 13,20 and 22 and the resistors 14 and 17 operate at a voltage potential of 1/2 Vcc.
In order for the conventional pulse width modulation amplifier described above to have good characteristics, the magnitude of amplification of the operational amplifier 1 would have to be increased to increase the magnitude of the negative feedback signals. However, since the operational amplifier 1 is a direct current amplifier, an increase in the amplification magnitude may generate an offset voltage, which is a deviation of the center voltage of the alternating signal to be amplified from the center voltage of the power source. Also, in the case of direct current amplification, direct current coupling is used for coupling to the next amplification stage. Thus, the offset voltage is transmitted to the next amplification stage, resulting in a significantly high offset voltage. The disadvantage of direct current amplification is that the generation of such an offset voltage may cause some harm and may be an inefficient use of a power source.
Another disadvantage of using direct current amplification is that the offset voltage cannot be corrected sufficiently by the operational amplifier. In particular, the low-pass filters in the negative feedback circuits 2 and 3 are direct-coupled to the operational amplifier 1. The offset voltage of the amplification stage after the comparator 5 is also applied to the operational amplifier. As a result, the operational amplifier cannot provide sufficient correction to the offset voltage.
A further disadvantage is attributable to the amount of noise in the signal of the conventional circuit described above. Noise may increase in such a circuit because the capacitors 13,20 and 22 and the resistors 14 and 17 are at the same common ground potential and the negative feedback circuits are direct-current-coupled to the operational amplifier 1. In such a configuration, the carrier of the output signal may leak to the operational amplifier.
A still further disadvantage of the conventional circuit is that the frequency of the carrier of the conventional pulse width modulation amplifier shown in FIG. 6 is higher than the audible range (for example 63 kHz). Moreover, the signals fed back from the output stage 7 include switching noise, introduced by the FET circuit 7 of the final output stage, and result in worse noise characteristics for the whole circuit.
The output noise characteristics of the conventional pulse width modulation amplifier with no input signal are shown in FIG. 7. Since the input voltage is 0, only the residual noise of the amplifier is analyzed. The plot in FIG. 7 is a frequency analysis of the voltage at the final output stage (both sides of the load 9 in FIG. 4) of the pulse width modulation amplifier shown in FIG. 4, including the operational amplifier 1 and negative feedback circuits shown in FIG. 6. It should be apparent that the noise in the output signal includes higher harmonic components and fractional harmonic components of the carrier, their complex, mixed, or modulated components, and the basic oscillation frequency 63 kHz. The audible range is also affected. The reason why these noise components are not eliminated by the low-pass filters in FIG. 6 is because the capacitors of the low-pass filters are grounded to the point of 1/2 Vcc due to the direct current coupling of the negative feedback circuits. To avoid noise, this point must be a perfect ground point. However, the impedance of the capacitor 13 of the power source filter, for example, is not actually infinitesimally small. Accordingly, the noise component generated at the final output stage is applied to the operational amplifier through the impedance of the power source and amplified.